THIS invention relates to coated abrasive particles and materials, and to a method of making them.
Abrasive grit, such as diamond and cubic boron nitride particles, are widely used in sawing, drilling, grinding, polishing and other abrasive and cutting applications. In such applications the grit is generally surrounded by a matrix consisting of metals such as Fe, Co, Ni, Cu and alloys thereof (metal bonds). Alternatively, resin (resin bond) or vitreous (vitreous bond) matrices can be used, the choice of matrix being a function of the particular application in which the abrasive is to be used.
The use of abrasive grit in the manufacture of abrasive tools is not without its problems. Vitreous bonded grinding wheel and tools containing ultrahard abrasive particles such as diamond and cubic boron nitride are widely used in general grinding operations. Typically the abrasive particles are held in a porous glass matrix. The tools are made by mixing or combining the ultrahard abrasive particles with glass frits and/or glass forming starting materials, compacting or forming a required shape for the grinding wheel or component of said wheel and then heat treating to a temperature sufficient for the glass to sinter to a desired degree such that a crushable porous matrix is obtained for the abrasive ultrahard particles.
There are several problems that limit the making and use of such abrasive wheels and articles. Firstly, in the case where diamond is the required ultrahard abrasive particle, the temperatures, heat treatment times, and furnace environments used or ideally required are such that significant degradation of the diamond particles can occur due to oxidation. It is well known that diamond oxidation reactions can detectably commence at temperatures as low as 550° C. in air and can become very rapid at temperatures exceeding 800° C. This limits the fabrication procedures to the use of inconvenient and sometimes expensive gaseous environments. Moreover the oxidation reactions of diamond being surface area dependent, become extremely rapid as the diamond abrasive particle size becomes small. This tends to limit the convenient use of diamonds in vitreous bonds to the coarser sizes such as about 100 to 150 micrometers (μm) in diameter although diamond sizes as fine as 1 to 10 μm may be desired for some applications.
It is often desired to incorporate organic compounds and agents into the glass compact so that controlled porosity may be generated by the pyrolysis and thermal degradation of such organics. Even though inert gas environments may be employed this pyrolysis of the organic components leads to highly oxidative products that can oxidize and damage the diamond abrasive particles.
Secondly, in the case where cubic boron nitride particles are the desired ultrahard abrasive, certain glass making components or compounds can inappropriately react with the cubic boron nitride leading to large amounts of gas evolution and foaming that can disrupt and damage the wheel or abrasive article. Examples of these glass components are alkali oxides, such as lithium oxide (Li2O), sodium oxide (Na2O) and potassium oxide (K2O). These components might be desired fluxing agents for the glass sintering and formation. Lithium oxide is known to easily react with cubic boron nitride at elevated temperatures with the evolution of nitrogen gas (N2). This gas evolution and resultant foaming can disrupt the fabrication of a vitreous bonded grinding wheel or article. The glass, vitreous bond choices are thus limited to those that do not contain significant amounts of compounds that can catastrophically react with cubic boron nitride.
This problem is also magnified as the cubic boron nitride particle size becomes smaller due to a large increase in surface area and resultant reactive surface and so there is also a tendency not to employ fine cubic boron nitride particle size distributions.
Thirdly, when mechanical mixtures of the ultrahard particles and the glass frits and/or glass starting material combinations are subjected to the glass sintering and formation conditions, bonding and keying of the abrasive particles into the vitreous matrix can be problematic due to inadequate wetting and contact between the abrasive particle and the glass.
Fourthly, often slow cooling rates are necessary during the manufacture of vitreous bonded tools to minimize cracking damage which can occur due to thermal expansion miss-match between the abrasive grains and the porous glass bond matrix.
Prior art exists where such problems are considered. EP 0,400,322 (also published as U.S. Pat. No. 4,951,427) claims abrasive particles, including diamond and cBN, having a refractory metal oxide substantially covering the surface of said particles. The metal oxide coats were claimed to provide means of substantially eliminating attack on cBN particles from the vitreous bond matrix of grinding wheels during their manufacture. The preferred refractory metal oxides were titania, zirconia, alumina and silica. The most preferred was titania.
The discussed method involves first applying a metal coat in an elemental form to the particles followed by converting said coat into oxides by heat treatment, preferably during firing in oxidizing atmospheres to produce grinding wheels. Although an alternative method for TiO2 involving forming a slurry with a metal organic compound, specifically tetra-isopropyl titanate, and then decomposing the said metal organic by heating is suggested in one of the examples, the example provided is non-enabling, has insufficient details and does not provide a means of coating individual fine particles in chosen phases of titania.
Moreover these procedures are inappropriate as the particle sizes of the desired starting constituents become finer, in particular for micron and submicron particulate materials and more particularly for nano-sized particulate materials, due to significant difficulties in coating each very fine particle evenly and the tendency to form agglomerates of the fine particles. Use of these procedures thus imposes limitations on coating particulate abrasive materials of fine sizes.
In U.S. Pat. No. 4,011,064 it is disclosed that rough granular adherent coats can be applied to cBN abrasive particles by milling the particles with metal compounds in ball mills in such a way that the metal compound may be smeared over the surfaces of the particles. The metal compound can then be decomposed by heating, between about 800 and 1400° C., in inert or reducing atmospheres to convert the metal compound into the metal. The exemplary metal compound disclosed is tungsten sulphide, WS2, resulting in a granular tungsten metal coat on 125 to 149 micron cBN particles.
It is expected that this method would be very difficult to apply to finer particles such as 10 micron or smaller and to not be applicable at all to sub-micron and nano-sized particles due to the requirement that the smeared material must itself be essentially comminuted to particles much smaller than particles to be coated. Moreover the metal compounds applicable in the method are restricted to those that may have appropriate mechanical properties for smearing.
Much of the prior art concerning the problems of abrasive particles to be incorporated into bonded tools and wheels deals with coating the abrasive particles in metals, ceramics and combinations of such materials. This body of prior art predominantly deals with various chemical vapour or physical vapour methods of generating such coats. Moreover it is expected that such techniques are limited and difficult to apply to fine abrasive particles, particularly those of micron, sub-micron and nano-sizes. It is expected that the prior art methods in general suffer from a deficiency in that it is difficult to expose each and every particle to identical reactive and coating environments and so variable coating from particle to particle inevitably ensues.
It remains that efficient methods of coating abrasive particles in materials that would serve to protect the abrasive from chemical attack by the many desired bond materials of grinding wheels and tools, be they vitreous bonds or metal bonds or other, is required. In particular, methods that enable fine sizes of abrasives of micron, sub-micron and even nano-size is required.